Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight substrate materials have been proposed.
Ceramic matrix composites (CMCs) are a class of materials that consist of a reinforcing material surrounded by a ceramic matrix phase. Such materials, along with certain monolithic ceramics (i.e. ceramic materials without a reinforcing material), are currently being used for higher temperature applications. Using these ceramic materials can decrease the weight, yet maintain the strength and durability, of turbine components. Furthermore, since these materials have higher temperature capability than metals, significant cooling air savings can be realized that increase the efficiency of a turbine engine. Therefore, such materials are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as airfoils (e.g. turbines, and vanes), combustors, shrouds and other like components that would benefit from the lighter-weight and higher temperature capability these materials can offer.
CMC and monolithic ceramic components can be coated with EBCs to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment. In dry, high temperature environments, silicon-based (nonoxide) CMCs and monolithic ceramics undergo oxidation to form a protective silicon oxide scale. However, the silicon oxide reacts rapidly with high temperature steam, such as found in gas turbine engines, to form volatile silicon species. This oxidation/volatilization process can result in significant material loss, or recession, over the lifetime of an engine component. This recession also occurs in CMC and monolithic ceramic components comprising aluminum oxide, as aluminum oxide reacts with high temperature steam to form volatile aluminum species as well.
Currently, most EBCs used for CMC and monolithic ceramic components consist of a three-layer coating system generally including a bond coat layer, at least one transition layer applied to the bond coat layer, and an optional outer layer applied to the transition layer. Optionally, a silica layer may be present between the bond coat layer and the adjacent transition layer. Together these layers can provide environmental protection for the CMC or monolithic ceramic component.
More specifically, the bond coat layer may comprise silicon and may generally have a thickness of from about 0.5 mils to about 6 mils. For silicon-based nonoxide CMCs and monolithic ceramics, the bond coat layer serves as an oxidation barrier to prevent oxidation of the substrate. The silica layer may be applied to the bond coat layer, or alternately, may be formed naturally or intentionally on the bond coat layer. The transition layer may typically comprise mullite, barium strontium aluminosilicate (BSAS), a rare earth disilicate, and various combinations thereof, while the optional outer layer may comprise BSAS, a rare earth monosilicate, and combinations thereof. There may be from 1 to 3 transition layers present, each layer having a thickness of from about 0.1 mils to about 6 mils, and the optional outer layer may have a thickness of from about 0.1 mils to about 40 mils.
Each of the transition and outer layers can have differing porosity. At a porosity of about 10% or less, a hermetic seal to the hot gases in the combustion environment can form. From about 10% to about 40% porosity, the layer can display mechanical integrity, but hot gases can penetrate through the coating layer damaging the underlying EBC. While it is necessary for at least one of the transition layer or outer layer to be hermetic, it can be beneficial to have some layers of higher porosity range to mitigate mechanical stress induced by any thermal expansion mismatch between the coating materials and the substrate.
Unfortunately, deposits of calcium magnesium aluminosilicate (CMAS) have been observed to form on components located within higher temperature sections of gas turbine engines, particularly in combustor and turbine sections. These CMAS deposits have been shown to have a detrimental effect on the life of thermal barrier coatings, and it is known that BSAS and CMAS chemically interact at high temperatures, i.e. above the melting point of CMAS (approximately 1150° C. to 1650° C.). It is also known that the reaction byproducts formed by the interaction of BSAS and CMAS are detrimental to EBCs as well as being susceptible to volatilization in the presence of steam at high temperatures. Such volatilization can result in the loss of coating material and protection for the underlying component. Thus, it is expected that the presence of CMAS will interact with the EBC, thereby jeopardizing the performance of the component along with component life.
Accordingly, there remains a need for novel environmental barrier coatings that provide CMAS mitigation capability for use in conjunction with ceramic substrate components.